A magnetic storage device using a magnetoresistive element as a storage element, a magnetic sensor, and a magnetic head for example are known as a magnetic unit.
As an example of the magnetoresistive element, a TMR structure having a tunnel insulating film provided between two magnetic substances will be described. FIG. 1 is a sectional view showing an example of a TMR element reported by Roy Scheuerlein, et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 128). This TMR element includes an antiferromagnetic layer 201, a pinned layer 202, a tunnel insulating layer 203, and a ferromagnetic free layer 204, which are laminted. The antiferromagnetic layer 201 is formed from FeMn (10 nm). The ferromagnetic pinned layer 202 is formed from CoFe (2.4 nm). The tunnel insulating layer 203 is formed from Al2O3. The ferromagnetic free layer 204 is formed from NiFe (5 nm). Conductive wiring lines are connected to the antiferromagnetic layer 201 and the free layer 204 such that voltage can be applied thereto. A magnetization direction of the pinned layer 202 is pinned to a certain direction by the antiferromagnetic layer 201. The free layer 204 is formed to easily be magnetized to a certain direction, where this magnetization direction can be changed by externally applying a magnetic field. Among lateral directions of a film of the free layer 204, a direction to which magnetization is easy is referred to as an easy axis and a direction which is perpendicular to the easy axis and to which magnetization is hard is referred to as a hard axis. When applying voltage between the free layer 204 and the pinned layer 202, an electric current flows through the tunnel insulating film 203, and a resistance value changes depending on the relationship between magnetization directions of the free layer 204 and the pinned layer 202. That is to say, the resistance is low when the magnetization directions are identical and the resistance is high when the magnetization directions are opposite to each other.
Next, a nonvolatile memory (magnetic storage device) using TMR elements as storage elements, will described. FIG. 2 is a perspective view showing an example of a nonvolatile memory reported by M. Durlam, et al., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, 2000 IEEE International Solid-State Circuits Conference DIGEST OF TECHNICAL PAPERS (p. 130). This nonvolatile memory 210 is provided with a pair of wiring lines intersecting on and under each of TMR elements 205 arranged in arrays. Upper electrodes 206 are connected to free layers of the TMR elements 205. Antiferromagnetic layers of the TMR elements 205 are connected to drains of transistors 208 provided in lower layers through third wiring lines 207. By letting the current flow through two wiring lines B (any of B1 to B4) and wiring line D (any of D1 to D4), a combined magnetic field is generated in the vicinity of an intersection and a magnetization direction of a free layer is set depending on a direction of the current. Consequently, a resistance value of the TMR element 205 can be changed. Data is read as follows. First, the transistor 208 connected to the TMR element 205 for read is turned to the on-state by a wiring line W. Next, voltage is applied from the wiring line B to the TMR element 205. Consequently, the current flows through the TMR element 205. The read is performed by evaluating the resistance value of the TMR element 205 with the flowing current.
In general, an antiferromagnetic substance used for an antiferromagnetic layer of the above-mentioned TMR element, is FeMn, IrMn, PtMn, and NiMn for example. Mn used for these materials is known to be a material which relatively moves easily at high temperature. In manufacturing processes of semiconductor devices, high-temperature treatment is often performed for the purpose of wiring lines formation and uniformity of transistor characteristics. When high-temperature treatment is performed during production of magnetic units however, Mn atoms diffuse to other magnetic substances and the like, and characteristics of magnetic units may be degraded. That is, conventional magnetic units have a problem that it is difficult to use high-temperature process.
As related art, Japanese Laid-Open Patent Application JP-P2005-85951A discloses a magnetic storage element and a magnetic memory. The magnetic storage device at least includes a storage layer which retains information depending on magnetization states of a magnetic substance, and a subsidiary magnetic substance layer of which magnetization state is changed due to external magnetic fields. The subsidiary magnetic substance layer includes a plurality of magnetic substance layers divided by nonmagnetic layers. In the subsidiary magnetic substance layer, magnetic interaction of antiparallel magnetizations is present between adjacent magnetic substance layers. The total amount of the magnetizations of the even-numbered magnetic substance layers of the subsidiary magnetic substance layer and the total amounts of the magnetizations of the odd-numbered magnetic substance layers of the subsidiary magnetic substance layer are approximately equal.
Japanese Laid-Open Patent Application JP-P2006-60003A (US2006038213) discloses a magnetic memory. The magnetic memory includes a magnetoresistive element including a free magnetic layer, a first wiring line extending in a first direction slanting with respect to an easy axis of the free magnetic layer, a second wiring line extending in a second direction perpendicular to the first direction, and a write circuit which writes data in the free magnetic layer by starting to supply a second write current to the second wiring line after starting application of a first write current to the first wiring line and before stopping the application of the first write current. The free magnetic layer includes first to N-th ferromagnetic layers (N is an integer, equal to or more than 4) and first to (N−1)-th nonmagnetic layers. The i-th nonmagnetic layer (i is any integer of 1 to N−1) among the first to (N−1)-th nonmagnetic layers is provided between the i-th ferromagnetic layer and the (i+1)-th ferromagnetic layer. The free magnetic layer is configured such that the strength of action to antiferromagnetically couple the j-th ferromagnetic layer and the (j+1)-th ferromagnetic layer (j is any integer of 2 to N−2) is greater than the strength of action to antiferromagnetically couple the first ferromagnetic layer and the second ferromagnetic layer.
Japanese Laid-Open Patent Application JP-P2006-73861A discloses a magnetic storage device. The magnetic storage device judges storage states based on the respective magnetization directions of a pinned layer and a free layer. The magnetic storage device includes an antiferromagnetic layer, the pinned layer which is formed on the antiferromagnetic layer and of which magnetization direction is pinned, a first nonmagnetic layer formed on the pinned layer, the free layer which is formed on the first nonmagnetic layer and of which magnetization direction is changed depending on external magnetic fields, and a metal film formed on the free layer. The metal film contains ruthenium and the thickness of the free layer is between 1.5 nm and 5 nm.
Japanese Laid-Open Patent Application JP-P2006-165265A discloses a storage element and a memory. The storage element includes a storage layer for retaining information depending on magnetization states of a magnetic substance. Magnetization pinned layers are provided above and below the storage layer through intermediate layers. The respective intermediate layers include insulating layers. In the magnetization pinned layers above and below the storage layer, the magnetization directions of the respective ferromagnetic layers nearest to the storage layer are opposite to each other. By flowing the current in a lamination direction, the magnetization direction of the storage layer is changed and information is recorded for the storage layer. Sheet resistance values of the two intermediate layers over and under the storage layer are different from each other.